Thermal spray processes have been employed broadly in numerous industries to apply protective coatings to a variety of substrates including metal, ceramic, plastic and paper. More recently, thermal spray methods have been utilized for the fabrication of high-tech composite materials as coatings and as freestanding near-net-shape structures. By heating and accelerating particles of one or more materials to form a high-energy particle stream, thermal spraying provides a method by which materials starting in wire or powder form may be rapidly deposited on a substrate. While a number of parameters dictate the composition and microstructure of the sprayed coating or article, the velocity and temperature of the particles as they impact the substrate are important factors in determining the density and uniformity of the deposit.
One prior art thermal spray technique is the utilization of a combustion flame to spray metals and other materials, in powder, wire or rod form onto a substrate. A mixture of a fuel gas such as acetylene and an oxygen-containing gas (oxy-fuel) are flowed through a nozzle and then ignited at the nozzle tip. The material to be sprayed is metered into the flame where it is heated and propelled to the surface of the substrate. The feedstock may comprise a metal rod or wire which is passed axially into the center of the flame front or, alternatively, the rod or wire may be fed tangentially into the flame. Similarly, a metal powder may be injected axially into the flame front by means of a carrier gas. Some powder only combustion flame guns utilize a gravity feed mechanism by which a powdered material is simply dropped into the flame front. Conventional flame spraying, however, is typically a low velocity thermal spray process in the subsonic range and usually produces coatings which have a high degree of porosity.
Another spraying technique is known as plasma spraying. Plasma spraying utilizes a high-velocity gas plasma to spray generally powdered or particular material onto a substrate. To form a plasma, gas is flowed through an electric arc in the nozzle of a plasma spray gun, causing the gas to ionize into a plasma stream. The plasma stream thus formed is at an extremely high temperature, often exceeding 10,000 degrees C. The material to be sprayed, typically particles of about 20 to 100 microns, are entrained in the plasma and may reach a velocity exceeding mach 1. While plasma spraying can produce high density coatings, it is a complex procedure which requires expensive equipment and considerable skill on the part of an operator for proper application.
In another thermal spray technique known from U.S. Pat. No. 3,546,415, an electric arc is generated in an arc zone between two consumable wire electrodes. As the electrodes melt, the arc is maintained by continuously feeding the electrodes into the arc zone. The molten metal at the electrode tips is atomized by a blast of generally cold compressed gas. The atomized metal is then propelled by the gas jet to a substrate forming a deposit.
Conventional electric-arc thermal sprayed coatings are generally dense and reasonably free of oxide, however, the process requires that two consumable wires (electrodes) be fed to the spraying apparatus resulting in several disadvantages. Firstly, the spray gun, which is often desired to be lightweight and easily handleable, is bulky and cumbersome due to the number of heavy electrical power cables and electrically insulated wire feed conduits which are required. Also, instabilities in the spraying process occur due to unavoidable irregularities which are inherent in the process of feeding two wires simultaneously and precisely to the arc zone in the spray gun. Such wire feed instabilities result in inconsistent characteristics in the coating characteristics often detrimental to coating quality. In addition, since molten particles are initially formed from both an anode and a cathode, two distinctive particle size ranges are produced from the two electrodes, which is not conducive to forming a uniform coating structure.
An arc spray system known from U.S. Pat. No. 4,668,852, provides variation of the arrangement of flowing the atomizing gas through and around the arc-zone. However, this technique only provides improved atomizing, at subsonic velocities and also does not resolve the difficulties incurred because of dual-wire feed instabilities.
To improve the velocity of the atomizing gas, an arrangement was devised as known from U.S. Pat. No. 4,788,402 whereby a plasma jet is formed in a manner so that the plasma gas is accelerated to sonic or supersonic velocity as it exits the anode nozzle of the plasma torch. Two wires are simultaneously fed into the exiting, high velocity ionized gas stream at acute angles to the axis of flow of this plasma stream. These two wires are electrically energized with respect to each other and an arc is formed between the two wires, through the ionized plasma jet stream. Such an arrangement provides a means of atomizing the metal particles formed from the melting ends of the two wires at very high velocities which could be sonic or even supersonic. However, such an apparatus still requires the simultaneous feeding of two consumable wires, not resolving the instability problems associated with the simultaneous feeding of two wires.
A method is known from U.S. Pat. No. 3,140,380 to form several plasma streams angularly displaced around a central axis. A single wire is fed along the central axis of this configured multi-plasma torch and is melted by the heat of the plasma and the molten particles are atomized and propelled to a substrate to form a coating by the combined plasma streams at the converging point of these multiple plasma streams. In this configuration, the heat available to melt this single wire is only that which is obtained by convection from the plasma stream. Also, the velocity of the converging plasma streams is relatively low and therefore the atomizing and propelling of the metal particles occurs at low velocity, thereby not producing high density pore-free coatings.
A single wire arc apparatus and process is known from U.S. Pat. No. 3,064,114 in which a single wire is fed through the central axis of a plasma torch. This wire acts as a consumable electrode being fed into an arc chamber. An arc is struck between this wire and a coaxially aligned outlet nozzle. Gas is fed into the arc chamber, coaxial to the electrode wire, where it is expanded by the electric arc and causes a highly heated gas stream carrying metal from the electrode tip to flow through the nozzle. This jet of gas coaxial to the electrode wire also assists in converting the electrode wire tip which is being melted by the electric arc, into a stream of fine metal droplets.
It is also known from U.S. Pat. No. 3,085,750 to provide a metal plate in the pathway of the torch to bend and direct the flow path of the heated gas stream. There are several deficiencies with this type of metal spray process. Firstly, the velocity of this process is subsonic, yielding deposits which are quite porous and resulting coatings which are comprised generally of relatively large particles. Additionally, a great deal of difficulty can be encountered in preventing build-up of the metal droplets on the walls of the outlet nozzle.
An arrangement is known from U.S. Pat. No. 4,370,538 in which a single wire is fed at an acute angle into a plasma stream internally within a dual stream torch. A transferred-arc is established between the cathode of the plasma torch and the wire anode, thereby melting the tip of the wire. Sufficient gas flow in the plasma stream is established to help to initially atomize the molten metal at the wire tip. This gas flow is at a relatively low velocity but at high temperature and the initially atomized particles are subsequently moved into a second, cooler, very high velocity gas stream for further atomization and acceleration. This second gas stream is derived from combusting an oxy-fuel mixture in a separate combustion chamber which is also an integral part of the proposed thermal spray torch. The hot combustion product, gas, is directed to coaxially combine with the plasma stream containing the partially atomized molten metal particles. One of the drawbacks of such an apparatus is the high degree of complexity of the equipment of combining several processes (plasma, combustion and wire arc) in one assembly along with the extremely fine balance of control of these three processes to get them to work in harmony with each other. In addition, the operation of such an apparatus is very expensive, requiring large consumptions of fuel gas and oxygen. Additionally, when the wire is fed at an acute angle into the plasma stream and an arc is struck between the wire tip anode and the cathode electrode of the plasma torch, secondary arcs (double arcing) can randomly occur between the wire and the anode nozzle of the internal plasma torch. Double arcing is a condition in which a shorter electrical path is found for the transferred-arc current to flow from the cathode electrode through internal arcing within the torch to a second arc which will form between a point on the outer surface of the torch and the wire. Such secondary (double) arcs can be destructive to the internal plasma torch and to the overall spray torch.
Another system is known from U.S. Pat. No. 4,604,306 in which two separate torches are employed, namely a plasma torch and a high velocity combustion torch. The plasma torch is described as a transferred-arc type torch in which an arc is struck between the cathode electrode of the torch and the tip end of a wire which is fed into an initially formed pilot plasma stream at an acute angle relative thereto. The molten particles which are partially atomized and accelerated from the transferred-arc zone are injected into a quiescent zone" formed at the exit of a high velocity oxy-fuel gun. The same disadvantages apply to this configuration as those pertaining to U.S. Pat. No. 4,370,538 previously mentioned. Since the wire is fed into the plasma stream at an acute angle, secondary arcing between the wire and the plasma torch anode pilot nozzle commonly occur due to this physical configuration resulting in damage and destruction of the plasma torch. In addition, the apparatus is complex requiring critical mechanical alignment between the plasma torch and oxy-fuel combustion torch and the process is very costly to operate.
One solution to the problem of secondary arcing is known from U.S. Pat. No. 4,762,977. A high velocity annular gaseous sheath is formed concentrically about the transferred-arc column to form an arc column guide restricting the arc column to within a region closely spaced radially from the axial extension of the nozzle, such that the arc column cannot penetrate this sheath. When the wire motion is stopped or the wire is withdrawn from the arc-zone, the arc to the tip of the wire is extinguished by the cold, high velocity annular gas flow. This solution to secondary arcing creates greater complexity and bulkiness to the spray apparatus as well as increasing the operating cost of such a system by requiring an additional high volume of high velocity flow of compressed air. In addition, it is not always useful to have a high volume of high velocity cold air impinging on the coating being formed on the substrate and can actually be detrimental to achieving the highest quality of coating characteristics.
Prior art thermal spray methods have been used to form composite materials by simultaneously spraying two or more distinct materials. Ceramic-ceramic composites, ceramic-metal composites known as "cermets", and metal-ceramic composites, known as "metal-matrix composites" have been formed as coatings and as freestanding near-net-shape articles. Materials may also be fabricated by forming a first particle stream using one spray gun and then combining the first stream with a particle stream from another gun to form a combined spray at the target surface.
A method of manufacturing a composite material by combined melt-spraying is known from U.S. Pat. No. 4,740,395. The use of a conventional single-wire combustion spray gun to melt and spray the main constituent metal onto a substrate is combined with an injection means which injects discontinuous fibers as a reinforcing material, together with compressed air into the metal spray wherein the discontinuous fibers are mixed into the metal spray. A composite material is thus formed on a substrate. The limitations of this type of technique are that the resulting deposits contain oxide formations surrounding each metal particle as well as a high degree of porosity resulting from the low-velocity nature of the process. Both of these factors result in deposits which do not have superior properties. In addition, the use of two separate spray guns to form composite coatings is difficult and unwieldy. It would therefore be desirable to provide a single spray gun which could be used to form composite materials such as metal-matrix composites which are high density essentially oxide free deposits.
Another means of thermal-spray forming composites such as metal-matrix composite materials as a coating or as freestanding near-net-shape articles, is described in currently pending U.S. patent application Ser. No. 07/247,024 of co-inventor Daniel R. Marantz. This device combines in a single apparatus a high velocity (operating in the trans-sonic range) oxy-fuel type spray gun with a two-wire electric-arc spray head. In this apparatus, the high velocity combustion products are directed at the arc-zone established between the two wires, where it acts to atomize and propel the molten metal formed in the arc, from the two wires to a substrate or article to be coated. Simultaneously, a powder feedstock of the reinforcement particles is fed into the combustion process within the high velocity oxy-fuel (HVOF) gun. This reinforcement particle, typically a refractory oxide or carbide, is heated and accelerated within the HVOF gun and is combined with the metal particles formed from the two-wire electric-arc. As the metal and reinforcement particles imbed themselves into the substrate, they are subsequently covered up by the splatting metal particles. This process produces a high density composite coating or bulk metal-matrix composite material.
There are several limitations and drawbacks to this process. First, since an oxy-fuel process is employed, large amounts of oxides are formed, surrounding each of the metal-matrix particles. This oxide formation weakens the interparticle bonding, thus forming a metal-matrix which is mechanically inferior to the wrought starting material. In addition, since the mechanism of reinforcement particle loading in the metal-matrix is based on the impact of the hot particle onto the coated surface, the hardness of the metal-matrix material employed plays an important role in how much, if any, reinforcement particle imbeds itself into the metal-matrix. Loading of reinforcement into the metal-matrix does not occur at all in harder materials such as steel and nickel based alloys. Only very low loading (less than 5%) are obtainable in medium hardness materials such as copper and its alloys. In softer materials, such as aluminum and aluminum alloys, reasonable loadings of 10 to 15 percent can be obtained. However this appears to be the limiting degree of loading obtainable.
Another method for thermal spray forming composites is known from U.S. Pat. No. 4,762,977. In this method a single wire is fed at an acute angle into a plasma stream in which a transferred-arc is established between the plasma stream and the tip of the wire. Simultaneously, a stream of carrier gas carrying powder feedstock is directed into the plasma upstream from the wire (between the plasma anode nozzle and the tip of the wire). The powder feedstock thus injected combines with the melted metal particle formed on the tip of the wire and each are propelled to the substrate all together, thus forming a composite structure in the resulting coating. However, since the powder particles and carrier gas are injected upstream from the tip of the melting wire, the cold carrier gas and entrained particles impinging into the transferred-arc causes the plasma stream to be cooled which, combined with the kinetics of interaction of the carrier gas stream and the plasma gas stream causes erratic arc conditions resulting in large non-uniformity in the resulting composite coating. Also, because of these interacting conditions and the resulting loading, the percent of secondary material included within the metal-matrix is limited to a low level.
Accordingly, it is desirable to provide a single wire electric spray gun which may be used to form composite materials such as metal-matrix composites and which achieves the benefits of supersonic plasma-arc powder and wire spraying.